Hi jd,
You only need to cross the threshold to be promoted to the next tier. After the promotion to Pro you're playing longer tees with longer approach shots in more difficult conditions. Most promotions are followed by an immediate ballooning of the player's scoring average while they adjust to their new surroundings. Once you are promoted you cannot be demoted back down to the previous tier, so you would just remain in the higher tier with a higher average while you sort everything out.